THE GLOBAL THERMOSTAT GOES UP

Britain has started sliding southwards, at about five miles per year. Within a couple of generations London will be somewhere well to the south of Paris, in distinctly warmer climes.

We are not actually on the move. But the man-made changes in climate now getting under way are equivalent to a gradual change in latitude. Climatologists tell us that the speed of change will accelerate.

It seems quite a pleasant notion at first - longer and hotter summers, fewer frosts.

But in changing the composition of the atmosphere, mainly by burning more and more coal, oil and gas, humanity has embarked on a colossal global experiment with no idea of the outcome. And if climate change does grave harm to our agriculture and economies, it seems certain to do far worse damage to wildlife.

The latest predictions from scientists are for a 2.5 degree centigrade increase in average global temperatures by the end of the next century as heat-trapping carbon dioixide and other gases from industry and agriculture build up in the atmosphere.

Sea levels will rise by around two feet as the warmer oceans expand. Those climate changes may not sound drastic, but they would be the most rapid since the last Ice Age ended about 10,000 years ago.

The temperature increase will be spread unevenly around the planet with the Arctic, for instance, warming up much more than the global average.

Along with the warming will come changes in rainfall and snowfall, wind patterns and ocean currents. Some places will get wetter, others may become deserts.

There are fears that major changes in ocean circulation could be triggered, which would in turn lead to much greater climate shifts on land.

Wildlife has had to cope with more drastic, but entirely natural swings in climate many times in prehistory as ice ages came and went. But these days, with more and more of the planet's surface intensively exploited by man, there is far less room for nature to manoeuvre.

Some species will be squeezed out of their homelands by climate change and have to colonise habitats elsewhere in order to survive. Separating the fragments of natural and semi-natural habitat which remain are huge stretches of farmland and urban development, which may prove quite impassable for many cold-loving plants and animal species as they seek cooler climes to the north.

In Britain the ptarmigan is one of several species confined to the tundra- like, semi-Arctic habitats found on our mountain tops. If these habitats disappear due to global warming then so does this particular bird.

Meanwhile, we can expect some animals and plants from warmer climes to set up home in Britain. There are signs of this happening already. A few insect and bird species from much further south in Europe, such as Cetti's warbler and the little egret, have begun breeding in Britain in recent decades.

What happens to rainfall is just as important as any changes in temperature. Even before the recent English drought years started in the late 1980s our steadily rising demand for water was denying nature its first share. Several wetlands were drying out, and several lowland rivers were running low or completely dry, because water companies and farmers were taking too much water.

The droughts we are now experiencing have made matters much worse for water-loving wildlife, and they may be part of man-made global warming - although it is too early to know for certain.

Reducing water wastage in the home and in leakages from the mains has now become a major campaign priority for the Wildlife Trusts and other conservation groups.

In a hotter, drier Britain, forest and moorland fires are likely to be more frequent and larger. In April this year a bone-dry Dartmoor National Park suffered a devastating blaze at Trendlebear Down.

It wiped out two square miles of heather and grassland, much of which was due to be declared a National Nature Reserve later this year. The fire wiped out the moor's only colony of Dartford warblers, a rare small bird, and a colony of the rare marsh fritillary butterfly.

As sea levels rise, the low-lying freshwater wetlands on the coast and the huge areas of mudflats lining estuaries will be gradually drowned. In Britain these areas are key feeding and breeding grounds for dozens of wildfowl and wading birds, some of them already quite rare.

The Norfolk Wildlife Trust knows all about the devastating effect of sea level rise. In February last year a combination of high tides and north-easterly gales breached sea defences, causing its freshwater Cley Marshes Nature Reserve on the Norfolk coast to be completely flooded by saltwater. The trust is fund-raising to try to right the damage, but if the these marshes became permanently saline then one of the largest remaining areas of habitat for one of the country's rarest and most unusual birds, the booming bittern, would be lost.

England's eastern and southern coastline was already sinking slowly before man-made climate change and the accompanying rise in sea level began. The invading sea poses a particular threat to coastal habitats and wildlife in Britain.

If we could instantly cut our emissions of "greenhouse gases" such as carbon dioxide and methane by two thirds then the man-made climate change would slow down and stop within a couple of decades.

But far from falling, emissions are rising at more than one per cent a year as consumption of fossil fuels keeps on rising along with economic growth and the industrialisation of poorer countries.

Meanwhile developed nations, which have pledged to freeze their rising annual emissions at the 1990 level by the year 2000, as a first step towards tackling the threat, are mostly failing to keep their promise.

So while you may hear plenty of talk about tackling global warming a lot of it, sadly, is hot air. Which is why organisations like Wildlife Trusts campaign hard for governments, industry and the public at large to use energy less wastefully and find alternatives to fossil fuels.